The term “acrylonitrile-butadiene rubber” or “nitrile rubber”, also named as “NBR” for short, shall be interpreted broadly and refers to rubbers which are copolymers or terpolymers of at least one α,β-unsaturated nitrile, at least one conjugated diene and, if desired, one or more further copolymerizable monomers.
Hydrogenated NBR, also referred to as “HNBR” for short, is produced commercially by hydrogenation of NBR. Accordingly, the selective hydrogenation of the carbon-carbon double bonds in the diene-based polymer must be conducted without affecting the nitrile groups and other functional groups (such as carboxyl groups when other copolymerizable monomers were introduced into the polymer chains) in the polymer chains.
HNBR is a specialty rubber which has very good heat resistance, an excellent resistance to ozone and chemicals and also an excellent oil resistance. The abovementioned physical and chemical properties of HNBR are associated with very good mechanical properties, in particular a high abrasion resistance. For this reason, HNBR has found wide use in a variety of applications. HNBR is used, for example, for seals, hoses, belts and damping elements in the automobile sector, also for stators, oil well seals and valve seals in the field of oil exploration and also for numerous parts in the aircraft industry, the electronics industry, mechanical engineering and shipbuilding. A hydrogenation conversion higher than 95%, or a residual double bond (RDB) content <5%, without cross-linking during the hydrogenation reaction and a gel level of less than about 2.5% in the resultant HNBR is a threshold that ensures said high-performance applications of HNBR and guarantees excellent processability of the final product.
The degree of hydrogenation of the copolymerized diene units in HNBR may vary in the range from 50 to 100% with the desired hydrogenation degree being from about 80 to 100%, preferably from about 90 to about 99.9%. Commercial grades of HNBR typically have a remaining level of unsaturation below 18% and a content of acrylonitrile of roughly up to about 50%.
It is possible to carry out the hydrogenation of NBR either with homogeneous or with heterogeneous hydrogenation catalysts. The catalysts used are usually based on rhodium, ruthenium or palladium, but it is also possible to use platinum, iridium, rhenium, osmium, cobalt or copper either as metal or preferably in the form of metal compounds (see e.g. U.S. Pat. No. 3,700,637, EP-A-0 134 023, DE-A-35 41 689, DE-A-35 40 918, EP-A-0 298 386, DE-A-35 29 252, DE-A-34 33 392, U.S. Pat. No. 4,464,515 and U.S. Pat. No. 4,503,196). Suitable catalysts and solvents for a hydrogenation in the homogeneous phase are also known from DE-A-25 39 132 and EP-A-0 471 250.
For commercial purposes the hydrogenation of NBR is performed in organic solvents by using either a heterogeneous or a homogeneous transition metal catalyst often based on rhodium or palladium. Such processes suffer from drawbacks such as high prices for the catalyst metals and the cost involved in catalyst metal removal/recycle. This has led to research and development of alternative catalysts based on cheaper noble metals, such as osmium and ruthenium.
Alternative NBR hydrogenation processes can be performed using Os-based catalysts. One catalyst well suited for NBR hydrogenation is OsHCl(CO)(O2)(PCy3)2 as described in Ind. Eng. Chem. Res., 1998, 37(11), 4253-4261). The rates of hydrogenation using this catalyst are superior to those produced by Wilkinson's catalyst (RhCl(PPh3)3) over the entire range of reaction conditions studied.
Ru-based complexes of the type Ru(X)Cl(CO)L2 with X meaning H or CH═CHPh are also good catalysts for polymer hydrogenation in solution and the price for Ru metal is lower. RuHCl(CO)L2 (L being a bulky phosphine) catalyst systems lead to quantitative hydrogenation of NBR as disclosed in Journal of Molecular Catalysis A: Chemical, 1997, 126(2-3), 115-131. During such hydrogenation it is not necessary to add a free phosphine ligand to maintain the catalyst activity. However, GPC results indicate that these catalysts cause a certain degree of cross-linking during hydrogenation and the HNBR obtained is prone to gel formation.
In EP-A-0 298 386 ruthenium complexes are used for the hydrogenation of nitrile rubber but various additives or ligands such as phosphines or carboxylic acids have to be added to avoid gel formation.
In EP-A-0 588 099 ruthenium based complexes are described of the type RuXY(CO)ZL2 where X, Y, Z and L can be halides, CO, carboxylates, or phosphines for the hydrogenation of nitrile rubber, however, the addition of water and certain additives is necessary to prevent excessive increase of molecular weight during hydrogenation. This, however, is not satisfying as the introduction of water causes corrosion in industrial scale equipment and additives are typically not desired as contaminants in the final rubber product. The amount of catalysts used is reported to be ca. 500 ppm or 0.05 parts by weight catalyst per 100 parts by weight of rubber.
Likewise, EP-A-0 588 098 and EP-A-0 588 096 use inorganic additives such as salts and acids like sulfuric and phosphoric acid for the same purpose together with traces of water which must be considered as being severely corrosive which in turn makes the use of expensive alloys for all the hydrogenation facility necessary. The amounts of catalyst needed vary from 923 to 1013 ppm based on Ru metal relative to rubber in EP-A-0 588 096 and approximately 100 ppm Ru metal in EP-A-0 588 098.
In EP-A-0 588 097 the catalysts RuCl2(PPh3)2 and RuHCl(CO)(PCy3)2 are used for the nitrile rubber hydrogenation in MEK solution. However, the obtained products are gelled unless a relatively high amount (ca. 5 phr) of ascorbic acid is added. In further examples the addition of various organic acids or dibasic acids is described but no solution is offered to the problem of a possible contamination of the final product with these additives. The amount of catalyst used is reported to be from 229 to 1052 ppm Ru metal per 100 parts by weight rubber.
EP-A-0 490 134 mentions a general problem associated with Ru-catalysts, namely the increase of viscosity during the hydrogenation. This problems tends to be dependent on the solvent used and has been associated to the interaction of two or more polymer molecules although concrete reaction mechanisms have not been established. Higher catalyst loadings also tend to cause higher viscosities or even gelling. It was found that amines added during the hydrogenation process in levels of 0.1 to 0.3 phr can avoid viscosity built-up. However, in view of large scale commercial processes one has to consider that additives can either built-up in the necessary solvent recycling processes or can be carried over into the product. Especially when using HNBR for peroxide cured rubber articles interference with the curing system can create serious drawbacks. Primary amines such as those found effective in EP-A-0 490 134 can reduce the cure state and as a consequence worsen the important compression set for example.
While some applications of HNBR may not be sensitive to the presence of additives such as for industrial applications there are also applications subject to government health and safety regulations which in general tend to favor cleaner polymers or polymers containing less additives. For practical reasons it is very desirable to run the same catalyst system in a plant for all designated products and a simpler catalyst system without additives may allow to produce products both for industrial and for regulated applications. Hence a catalyst system which does not need to be accompanied with additives would be much more desirable from the process and the rubber users point of view. It is furtheron desirable that a catalyst allows a good control of molecular weight during hydrogenation and a reproducible and sufficiently heat stable polymer. Additionally higher catalyst efficiencies are also desirable as precious metals such as Palladium, Rhodium, Iridium, Platinum and Ruthenium are expensive.
Various Ruthenium catalyst compositions have been proposed which can be used for both, the metathesis as well as the hydrogenation of nitrile rubber.
In WO-A-2005/080456 the preparation of HNBR having low molecular weights and narrower molecular weight distributions than those known in the art is carried out by simultaneously subjecting the nitrile rubber to a metathesis reaction and a hydrogenation reaction. The reaction takes place in the presence of a Ruthenium- or Osmium-based penta-coordinated complex catalyst, in particular 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene) (tricyclohexyl-phosphine) ruthenium (phenylmethylene) dichloride (also called Grubbs 2nd generation catalyst). However, WO-A-2005/080456 does not provide any disclosure or teaching how to influence these two simultaneously occurring reactions, i.e. metathesis and hydrogenation or how to control the activity of the respective catalysts in this regard. Provided these catalysts can be used in hydrogenation processes with controlled or suppressed metathesis activity and therefore provide controlled or suppressed molecular weight reduction they still possess structures of considerable complexity. In particular the presence of substituted benzylidene ligands adds further synthesis steps to the catalyst manufacture and thus increases the manufacturing costs.
WO-A-2011/029732 also discloses an alternative process for subjecting a nitrile rubber in the presence of hydrogen to a combined and simultaneous metathesis and hydrogenation reaction in the presence of specifically defined penta-coordinated Ruthenium- or Osmium-based catalysts in order to prepare hydrogenated nitrile rubbers having low molecular weights and a narrow molecular weight distribution. In particular the so-called Hoveyda catalyst and the Arlt catalyst as shown hereinafter are used and turn out to be efficient at levels of 0.041 phr and 0.045 phr, respectively, in typical nitrile hydrogenation experiments.

Still, these catalysts contain two relatively complex carbene ligands, namely a benzylidene ligand and an unsubstituted or substituted imidazolidinyl ligand bound to the Ru-atom like a carbene complex. Apart from the challenging synthesis procedures the metathetic activity of the catalysts during hydrogenation is not always desirable.
WO-A-2011/023788 also discloses a process for subjecting a nitrile rubber in the presence of hydrogen to a combined and simultaneous metathesis and hydrogenation reaction in the presence of specifically defined hexa-coordinated Ruthenium- or Osmium based catalysts to prepare HNBR having lower molecular weights and narrower molecular weight distributions than those known in the art. Such process is performed by using a catalyst of general formulae (I) to (III)
where    M is ruthenium or osmium,    X1 and X2 are identical or different ligands, preferably anionic ligands,    Z1 and Z2 are identical or different and neutral electron donor ligands,    R3 and R4 are each independently H or a substituent selected from the group consisting of alkyl, cycloalkyl, alkenyl, alkynyl, aryl, carboxylate, alkoxy, alkenyloxy, alkynyloxy, aryl-oxy, alkoxycarbonyl, alkylamino, alkylthio, arylthio, alkylsulphonyl and alkyl-sulphinyl residues, each of which may optionally be substituted by one or more alkyl, halogen, alkoxy, aryl or heteroaryl moities, and    L is a ligand.
Grubbs III catalyst as shown below is used as preferred catalyst, which contains both, a benzylidene ligand and a carbene ligand of the NHC type and thus these catalysts are not as easy accessible as desired.

WO-A-2011/079799 discloses a broad variety of catalysts the general structure of which is shown hereinafter

It is stated that such catalysts can be used to provide modified nitrile butadiene rubber (NBR) or styrene-butadiene rubber (SBR) by depolymerisation. It is further stated that the catalysts can be used in a method of making a depolymerized HNBR or SBR rubber by adding one or more of those catalysts first to carry out depolymerisation of NBR, followed by adding hydrogen into the reactor under high pressure for hydrogenation. In another embodiment it is disclosed to prepare HNBR by adding hydrogen under high pressure first, then followed by adding one or more of the above catalysts. However, WO-A-2011/079799 does not provide any disclosure or teaching how to influence the different catalytic activities of the catalysts for depolymerisation (metathesis) and hydrogenation. It is accepted that while hydrogenation takes place simultaneously metathesis leads to a degradation of the molecular weight in uncontrolled manner.
A number of further references describe the use of metathesis catalysts in two step reactions starting with a ring-opening metathesis polymerisation (ROMP) first which is followed by a hydrogenation reaction (so called “tandem polymerization/hydrogenation reactions”).
According to Organometallics, 2001, 20(26), 5495-5497 the metathesis catalyst Grubbs I catalyst, namely RuCl2(PCy3)2-benzylidene can be used for ROMP of cyclooctene or a norbornene derivative first, then followed by a hydrogenation of the polymers. It is reported that the addition of a base like triethylamine increases the catalytic activity in the hydrogenation reaction. Hence, this catalyst still has the disadvantage of needing an additive and in general, this catalyst being the first generation of the Grubbs catalysts has an overall too low activity.
J. Am. Chem. Soc 2007, 129, 4168-9 also relates to tandem ROMP-hydrogenation reactions starting from functionalized norbornene monomers and compares the use of three Ruthenium-based catalysts, i.e. Grubbs I, Grubbs II and Grubbs III catalysts in such tandem reactions. It is described that the Ruthenium-based catalyst on the end of the polymer backbone is liberated and transformed into a hydrogenation-active species through reaction with H2, a base like triethylamine, and methanol. Hence the reaction suffers the same drawback as disclosed in Organometallics, 2001, 20(26), 5495-5497.
Inorg. Chem 2000, 39, 5412-14 also explores tandem ROMP polymerization/hydrogenation reactions and focuses on the mechanism of the hydrogenolysis of the ruthenium-based metathesis catalyst Grubbs I. It is shown that such catalyst is transformed into dihydride, dihydrogen and hydride species under conditions relevant to hydrogenation chemistry. However, there is no disclosure at all about hydrogenation of unsaturated polymers.
Summarizing the above it becomes clear that    (1) up to now, hydrogenation catalysts which are very active for the selective hydrogenation of nitrile rubbers are known and Rh- and Pd-based catalysts are already used in industrial hydrogenation processes; however, cheaper Ru-based hydrogenation catalysts are still facing the gel formation problem when used for NBR hydrogenation.    (2) some Ruthenium based complexes which are designed for high activity in metathesis reactions allow the degradation of nitrile rubber by metathesis first which may then be followed by a hydrogenation of the degraded nitrile rubber to afford hydrogenated nitrile rubber; if the same catalyst is used for metathesis and for hydrogenation, such catalysts are highly active for NBR metathesis while not so active for NBR hydrogenation at the same time; and    (3) catalysts which possess both, i.e. catalytic activity for metathesis and hydrogenation, could not be used in a controlled manner so far.
In four patent applications of the same applicant not yet published novel catalyst compositions based on Ruthenium have been described as obtainable by contacting complex catalysts originally disposing of both, metathetic and hydrogenation activity, with different co-catalysts, thereby controlling or even destroying the metathetic activity in order to selectively hydrogenate nitrile rubbers. However, as the preparation of such catalyst compositions includes an additional preparation step there is still a need to provide further improved yet simple enough catalysts allowing a selective hydrogenation of nitrile rubber at low catalyst concentrations.
EP-A-0 298 386 discloses a simple catalyst structure with a so called N-heterocyclic carbene ligand as it is e.g. present in Grubbs II or Grubbs III catalyst or Grubbs-Hoveyda catalyst.
Ruthenium complexes as shown in the below formula have been prepared for the first time and tested for 1-hexene hydrogenation by Nolan et al (Organometallics 2001, 20, 794) and the RuHCl(CO)IMes(PCy3) catalyst was found to be less active than the simpler RuHCl(CO)(PCy3)2 wherein Cy means cyclohexyl. This reference, however, does not provide any disclosure, hint or teaching whether such complexes may be also used for hydrogenating polymers, in particular nitrile rubbers and if the use of RuHCl(CO)IMes(PCy3) as catalyst compared to the use of RuHCl(CO)(PCy3)2 as catalyst has any impact on or benefit for the physical properties of any hydrogenated nitrile rubbers obtained thereby.
Fogg et al. (Organometallics 2005, 24, 1056-1058) prepared the complexes RuHCl(CO)(PPh3)NHC with NHC=IMes or SIMes by reaction of the precursor RuHCl(CO)(PPh3)3 with the respective NHC-ligand.

Hydrogenation trials with cyclooctene revealed a good hydrogenation efficiency for this catalyst with NHC=IMes (IMes being N,N′-bis(mesityl)imidazol-2-ylidene) but as a side reaction also ca. 20% of a ROMP-polymer were found which indicates that this catalyst or a derived active species have metathesis activity. This in turn implies that a hydrogenation of diene-polymers would be accompanied by molecular weight degradation as this process proceeds through metathesis steps. While this is beneficial for the purpose of generating lower molecular weight rubbers it is a serious drawback for a controlled preparation of hydrogenated polymers without molecular weight reduction which are necessary for many high end rubber parts. Contrary thereto no ROMP-polymerization was observed with cycldodecene as substrate to be hydrogenated. This different behaviour depending on the substrate does not allow a person skilled in the art to draw any conclusion or make any prediction about those catalysts' behaviour. Organometallics 2005, 24, 1056-1058, however, does not provide any disclosure, hint or teaching whether such complexes RuHCl(CO)(PPh3)NHC may be also used for hydrogenating polymers, in particular nitrile rubbers, and if the use thereof as catalyst has any impact on or benefit for the physical properties of hydrogenated nitrile rubbers obtained thereby.
Ruthenium catalysts with NHC-ligands have been found to be very active in general but their use in direct hydrogenation has been limited as the bond between the metal and the carbon atom of the NHC ligand has been found to be susceptible to reductive elimination under reducing conditions as stated by Albrecht et al (Organometallics 2009, 28, 5112-5121). The authors further report on NHC-ligands which possess additional chelating groups which can be hemilabile in order to prevent the elimination of the NHC groups from the metal coordinating sphere. This approach worked in some cases for the styrene hydrogenation but has not been tested in the hydrogenation of unsaturated polymers. It was also found that an olefin as chelating group was in fact hydrogenated in the early stage of the hydrogenation again leaving the NHC-ligand as a mono-dentate ligand which finally was completely split-off from the complex as imidazolium-salt and the metal complexes ended up as a black solid at the end to the run.
Thus, the available academic literature actually discourages the use of Ruthenium catalysts with mono-dentate bonded NHC-ligands for the purpose of demanding hydrogenation processes.
In Macromolecular Rapid Communications 19, 409-411 it is further disclosed that nitrile rubber may be hydrogenated under two-phase conditions using RuHCl(CO)(PCy3)2 as catalyst which is immobilized in 1-butyl-3-methylimidazolium tetrafluoroborate molten salt.
Fogg et al (Organometallics 2009, 28, 441-447) tested the hydrogenation performance of catalysts with the formulae RuHCl(H2)(PCy3)(L) and RuHCl(CO)(PCy3)(L) wherein L=P(Cy)3 with Cy being cyclohexyl or L=IMes with IMes being N,N′-bis(mesityl)imidazol-2-ylidene for various substrates such as styrene and allyl-benzene as well as for polymers obtained by ROMP of norbornenes and derivatives thereof. Comparing the Ruthenium complexes with and without a NHC-ligand one cannot find a clear advantage for those ones with an NHC-ligand. The trials were conducted under moderate conditions such as room temperature and up to 55° C. with relatively high catalyst loadings. The hydrogenation of polymeric substrates is much more demanding compared to the hydrogenation of small molecules and results in substantially lowered turnover frequencies as well as conversions despite high/higher catalyst loadings and in the necessity of increased reaction times. Under these reaction conditions the above mentioned side-reaction of NHC-ligands are even more likely to occur.
Albrecht et al (European Journal of Inorganic Chemistry (2011), 2011(18), 2863-2868) even found that Ruthenium complexes with NHC-ligands and a cumene ligand tends to hydrogenate nitrile groups which is a reaction very undesirable and a supposed cause for gelling.
In view of these obstacles there was still a need for an improved hydrogenation of nitrile rubber with simple Ruthenium based complexes which do not require further additives. Such process should proceed in a controlled manner, i.e. without a simultaneous molecular weight degradation due to a metathesis reaction. Such process should further provide access to hydrogenated nitrile rubber having in particular medium to high molecular weight and with a range of Mooney viscosities (ML1+4@100° C.) of from 60 to 130 Mooney units and should be efficient in that low amounts of catalyst already give the necessary high conversions in short reaction times. According to prior art a catalyst removal or recycle step is so far required after the hydrogenation to remove undue high residual catalyst contents. Hence the novel process to be provided should preferably represent a leave-in-catalyst technology.